Windell Oskay of Evil Mad Scientist Laboratories removed the RFID circuit in a MasterCard PayPass and placed it into a Hello Kitty Pez dispenser. He calls it "Hello Credit."
One interesting thing is that the circuit board is incredibly thin -- approximately 1/4 the thickness of standard 1/16" circuit boards, which happens to be pretty close to the thickness of a standard credit card. It's flexible and fairly translucent. You do need to be a little bit careful because ceramic capacitors (like the ones on this board) have a tendency to break if you bend the board that they are soldered to.Link (Via Neatorama)
Of course, we can now give our RFID element any kind of a case mod that we see fit. One thing that would be easy to do is just to embed it in the side of a wallet. You could also embed it in any one of the millions of things that have become USB thumb drives in the last few years.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects